Hot carrier metal base transistor having a p-type emitter and an n-type collector

ABSTRACT

A p-type material comprising the emitter region of a metal base hot carrier transistor with n-type material comprising the collector to provide a p-metal-n hot carrier transistor. Hot holes injected through the emitter-base barrier give rise to hot electrons having sufficient energy to pass through the basecollector barrier.

United States Patent [191 Shim et al.

[ 1 Aug. 7, 1973 HOT CARRIER METAL BAsI: TRANSISTOR HAVING A P-TYVPEEMITTER AND AN N-TYPE COLLECTOR Inventors: George A. Shirn,Williamstown,

Mass; Norman Carlile Miller, Shaker Heights, Ohio; William L. Patterson,Williamstown, Mass.

Assignee: Sprague Electric Company, North Adams, Mass.

Filed: Mar. 1, 1972 Appl. No.: 230,597

U.S. C1..-. 317/235 R, 317/234 R, 317/235 UA,

317/235 AQ Int. Cl. H011 1,1/00, H011 15/00 Field of Search; 317/234, 5,5.2,

[56] References Cited UNITED STATES PATENTS 3,424,627 1/1969 Michel etal. 317/235 UA 3,424,890 1/1969 Ruyven 317/235 UA 3,439,290 4/1969Shinoda 317/235 UA 3,582,410 6/1971 Lachapelle 317/235 UA PrimaryExaminer--.lohn W. Huckert Assistant Examiner-Andrew .1. JamesAttorney-Vincent H. Sweeney et al.

[57] ABSTRACT A p-type material comprising the emitter region of a metalbase hot carrier transistor with n-type material comprising thecollector to provide a p-metal-n hot carrier transistor. Hot holesinjected through the emitter base barrier give rise to hot electronshavingsufficient energy to pass through the base-collector'barrier.

1 Claim, 6 Drawing Figures 1 IIoT CARRIERIMETAL use TRANSISTOR HAVING AP-T'YPF. EMITTsIt AND AN N-TYPE COLLECTOR BACKGROUNDOF THE INVENTIONThis invention relates to metal base transistors of the hot electrontransistor type and more particularly to a metal'base hot-carriertransistor having an emitter of p-type conductivity.

Presently metal basen-m-n transistors are high fre quency devices butpresent certain shortcomings'in production. v v g It isan object of thisinvention to provide a metal base hot carrier transistor having improvedcharacteristics.

It is another objectofthis invention to provide a ptype emitter-base hotcarrier transistor which can be readily produced.

SUMMARY OF THE. INVENTION A metal base p-m-n hot carrier transistorwhich ina signal circuit diffuses hot holes from the p-type'emitterregion into the metal base/These holes give rise in the metal base toelectrons which can cross the basecollector barrier which is between themetal and an ntype collector. The transistor is connected in the signalcircuit to forward bias the emitter-base barrier and the collectorregion is positive with respect tothe base.

' BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION OFVTHE INVENTIONFIG. 1 illustrates a device of this invention formed on a body of nnepitaxialstructure to provide a collector 10. On a major surface onwhich is first epitaxially grown a layer of n-type silicon suitable forcollector 10, a suitable molybdenum layer is deposited to provide metalbase 12. It will be understood that molybdenum is referred to by way ofillustration and is preferable because it does not alloy with siliconattempera- Iui'es ordinarily used in manufacture. Next'a layer of P-typesilicon is deposited over the base 12 to provide anemitter 13. Asillustrated in FIG. 1 the base 12 and the emitter 13 are provided withpads 14 and 15 respectively. The pad 14 is provided with contact 16 andthe pad 15 is provided with contact 17.

In a specific example, a process for producing the transistor of thisinvention may use a starting material formed of suitable bodyofsingle'crystal 1 11 silicon of 0.01 ohm/cm n-type silicon with anepitaxial layer formed thereon with about 3 microns of about 1 ohm/cmn-type silicon in standard epitaxy equipment. A collector 10 thusprovided has a major surface 11. On the surface 11 there is provided thebase 12 by sputtor. Theemitter-base barrier is indicated as C-C. In-,

teringmolybdenum through a mask under suitable conditions, such as, forexample, a target voltage of 1,500, pressure 1.5 p. argon. The mask ismovable 1.6 mm/hr. target current 1.2 ma and'the substrate temperature300 C. Deposited under these conditions a molybdenum base 12 depositedon the collector surface 11 is highly oriented, and about angstromsthick. The mask motion across the collector surface is stopped but thedeposition continued at the end of the layer so as to build up a thickarea of the molybdenum base pad 14. The particular resistivity of thecollector 10 is that which provides rectifying contact with themolybdenum base 12.

Nextthe mask is returned across the collectorsurface and depositedmolybdenum layer to the end of base 12 opposite the built-up pad 14.Silicon is sputtered through the mask on top of the molybdenum base 12.The mask is again moved across the surface to form the emitter l3 andstopping short of the molybdenum pad 14. The silicon deposition iscontinued with the mask stationary to build up a thick area for 'anemitter pad 15. Suitable silicon sputtering conditionssimilar to thosenoted above for molybdenum are established with a silicontarget of; forexample, 1 A inch diameter by '16 inch thickness and of l00 p-type 0.01ohm/cm silicon.

The desired shape of the p-metal-n transistor device is then defined byetchinginto a strip using standard masking and etching techniques. Thestrip is 0.003 inch wide by 0.030 inch long incorporating the base 12,base pad 14, emitter 13, emitter pad 15 and a section of the ann-metal-n hot electron transistor in which energy level is plotted asordinate against distance as abscissa. Dash line 20 indicates the Fermilevel, 15,, solid line 21' represents the top of the valence band, solidline 22 represents the bottom of the conduction band. In FIG. 2,'theemitter material is n-type conductivity, the base is, metal and thecollector is n-type conductivity. The emitter barrier is indicatedby'A--A. Electrons can diffuse over the forward biased emitter barrierA-A.' These electrons are at an energy several 'kT above the Fermilevel, E in the metal as indicated by da. After dif-' fusing across themetal basejthe electrons are collected a by the collector field, as thehot electrons have suffi The energy diagram shown in FIG. 3 depicts adevice in which the emitter material is a p-type'semiconducstead ofelectrons,-holes diffuse across the emitter-base barrier C-C into themetal of the base. In the base the holes are below the Fermi level. Thehole-energy is' One explanation for the energy transfer from hole-toelectron is that of an electron-hole-phonon interaction of the typedescribed in .I.M. Ziman, Electrons and Phonons," Oxford Univ. Press.1960, London. It is related to the metal composition.

The energy diagram shown in FIG. 4 also depicts the p -metal-n hotelectron transistor. Hot holes from thep-type emitter penetrate into thecollector a small dispass through tance as determined by the energy ofthe holes and the retarding field. A certain number of holes being inthe collector region at all times, a space charge is produced in thecollector near the metal-collector interface. The holes penetrate until,their kinetic energy being converted to potential energy, the collectingfield in the collector region returns the holes to the base.

However, the steady state condition maintains sufficient positive spacecharge so that the potential near the barrier D" is pulled down enoughto allow thermal electrons in the metal base to tunnel across thebarrier D'"". Thus the steady state positive space charge provides acollector depletion layer for receiving tunnel electrons from the metalbase.

The effect of the hot holes transfer is illustrated by the resultsachieved with the circuit shown in FIG. 5 and the curves of FIG. 6. Thestrip device of FIG. 1 described above was connected in a common baseconnection as shown by the equivalent circuit of FIG. 5. The collectorcurrent density and voltage are plotted respectively as the ordinate andthe abscissa of the graph of FIG. 6.

The curves 23, 24 and 25 show that an increase in collector current withemitter drive can be obtained. The merit of this invention can be seenfrom the results shown by the curves 23, 24 and 25. The current densityclearly cannot be obtained with a passive network. It may be said,therefore, application of this invention enables transistor action.

in the device of FIG. 1 the materials may be those suitable for a hotelectron device such as thin aluminum for the metal base andsemiconductive silicon or germanium for the emitter and collector. Thedevice has high frequency capabilities. Also the p-type emitter providesadvantageous production possibilities in comparison with the n-m-ndevice.

What is claimed is:

l. A metal base hot hole device consisting essentially of a p-typesilicon emitter layer having a sputtered structure on the surface of athin molybdenum layer base having a highly oriented single crystalstructure on a n-n epitaxial body as a collector having an epitaxialstructure to form metal-semiconductor junctions between the emitter andthe base and the body as the collector, and circuit means for biasingthe emitter-base junction in a forward direction, the holes in theemitter, the thickness of the metal layer and the potential in thecircuit means cooperating to cause holes to move across the junction ofthe p-type sputtered silicon emitter with the highly oriented thinmolybdenum base, and the base and the collector being connected to causethe holes under kinetic energy in the molybdenum to undergo energytransfer from hole-to-electron, the electrons in the base being hot, andunder the said basecollector connection penetrating and crossing thebasecollector junction.

* i i i

1. A metal base hot hole device consisting essentially of a ptypesilicon emitter layer having a sputtered structure on the surface of athin molybdenum layer base having a highly oriented single crystalstructure on a n-n epitaxial body as a collector having an epitaxialstructure to form metal-semiconductor junctions between the emitter andthe base and the body as the collector, and circuit means for biasingthe emitter-base junction in a forward direction, the holes in theemitter, the thickness of the metal layer and the potential in thecircuit means cooperating to cause holes to move across the junction ofthe p-type sputtered silicon emitter with the highly oriented thinmolybdenum base, and the base and the collector being connected to causethe holes under kinetic energy in the molybdenum to undergo energytransfer from hole-to-electron, the electrons in the base being hot, andunder the said basecollector connection penetrating and crossing thebase-collector junction.